In high frequency communications, fading and channel noise may vary in time and be different for different frequencies within a band of frequencies. Thus, within the band, some of the frequencies will be usable and others unusable at any given instant in time. Which frequencies are actually most useful is unpredictable. At any given instant in time and within a given high frequency band, there will always be a maximum usable frequency, which is an unpredictable variable defining a frequency neighborhood in which reliable communication may best be carried at that instant in time. The multi-path fading effects at high frequencies can be very severe and quickly varying. Furthermore, as the most usable frequency deviates away from a fixed frequency, the multi-path conditions on that frequency will vary. Typically, the most usable frequency will drift among all the frequencies in the high frequency band of interest.